(a) Field of the Invention
The present invention relates to a display device, and more particularly it pertains to a colored-light display device utilizing light-emitting diodes serving as light source constituting elements.
(b) Description of the Prior Art
Most of the colored-light emitting display devices, such as traffic signal devices and railroad signal devices, employ incandescent lamps serving as light sources, and also color filter means for coloring the display lights. For example, a city road traffic signal device comprises a plurality of lamp units, each comprising a reflecting mirror, an incandescent lamp disposed on the reflecting mirror to serve as the light source, and a colored front cover lens arranged in the foreground of the reflecting mirror. The reflecting mirror, the light source and the cover lens are arranged at predetermined positions within a housing. City traffic continues day and night throughout the year, and hence the city traffic signal devices mostly are required to be operated day and night for the control and security of the traffic. This means that the incandescent lamps serving as the light sources of such traffic signal devices should be turned on and off frequently day and night. Furthermore, traffic signal devices which are installed outdoors are subjected directly to varying severe environmental conditions such as temperature and weather. Therefore, there are many factors that can cause malfunctions of the signal device, including disconnection of lamps. Thus, sufficient care, and hence considerable cost, need to be paid for the maintenance of the system. Furthermore, since the incandescent lamp supplies only white color lights, a coloring filter such as a colored cover lens is required for coloring the display lights. Filtering away of those unnecessary lights other than the light of a desired color such as green, yellow or red results in a reduction in the intensity of illumination or brightness and also in a low efficiency of conversion of electric power to a usable light energy. To compensate for such low efficiency, it is inevitable to use a light source of high wattage for obtaining sufficient brightness of display. A large power consumption, however, contributes to a large heat generation, and leads to a remarkable rise of the temperature within the lamp housing. Therefore, consideration should be paid not only to finding means to cope with the variations in the environmental conditions, but also to find means to cope with the variations of the temperature within the lamp housing. Such being the actual circumstances, the overall structure of signal devices for controlling the city traffic has tended to be large in size and complicated in mechanism. Signal devices for controlling the railway traffic have similar problems also.
Furthermore, another problem comes to the fore in case a signal lamp device which is provided with a coloring filter at the front cover or foreground surface of the device is installed at such location where the coloring filter is subjected to direct irradiation of intensive lights such as the sunlight. Such sunlight which is transmitted to the device through the coloring filter is subjected to being colored through the filtering function thereof, and is reflected by the reflector member or like member, and is caused thereby to emit outwardly of the device through this filter. Such reflected colored light from the signal lamp device could tend to give false indication to the viewer as if the signal which, in reality, is turned off looks like working. This kind of false indication cannot be prevented in those conventional signal devices having such structure as stated above. Such false indications could lead to traffic accidents. Among the conventional traffic signals, shadow signal lamp devices intended for pedestrians such as a device which displays a shadow figure of a moving person against a blue-color background or a shadow figure of a waiting or standing-still posture on a red-color background tend to have the above-said problems especially, because of the relatively low degree of brightness of the colored display surface and/or because of the white or semi-transparent shadow figures provided on the surface of the device. Similar signal lamp devices include a colored "GO" and "STOP" sings.
As will be understood from the foregoing statement, most of the inconveniences and drawbacks of those conventional colored-light emitting display devices may be attributed to the use of a combination of an incandescent lamp and a coloring filter.
For the purpose of colored-light display, the employment of light-emitting diodes is advantageous as compared with the incandescent lamps, with respect to such aspects as low power operation, negligible heat generation, long service life and high luminous effciency. Further development of colored-light display devices using light-emitting diodes have been demanded.